Cao Mao (241–260), courtesy nameYanshi, was the fourth emperor of the state of Cao Wei during the Three Kingdoms period. He was a grandson of Wei's first emperor Cao Pi. Described as intelligent and studious, Cao Mao made repeated attempts to seize back state power from the regents Sima Shi and Sima Zhao but failed. He was killed in an abortive coup d'état against Sima Zhao.

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Cao Mao was a son of Cao Lin (曹霖), the Prince Ding of Donghai (東海定王) and a son of Cao Pi. In 244, at the age of three, in accordance with Cao Wei's regulations that the sons of princes (other than the first-born son of the prince's wife, customarily designated as the prince's heir) were to be instated as dukes, Cao Mao was granted the title of "Duke of Gaogui".[1] Cao Mao's father died in 249 when he was eight. His older brother, Cao Qi (曹啟), succeeded their father as Prince of Donghai.

By 254, state power was in the control of the Sima clan, whose patriarch Sima Yi had seized power from Cao Fang's regent Cao Shuang in 249.[1] After Sima Yi's death in 251, the Sima clan was led by his son Sima Shi.[1] In 254, after falsely accusing the emperor's father-in-law Zhang Ji (張緝) and Zhang's associates Li Feng and Xiahou Xuan of treason, Sima Shi had them and their clans executed, and when Cao Fang considered a coup against the Simas later that year, Sima Shi had him deposed.[1]

It was at this time that Cao Fang's stepmother Empress Dowager Guo made a last-ditch attempt at preserving Cao Wei's imperial authority, by injecting herself into the process of selecting the next emperor. When Sima Shi notified her that he intended to make Cao Pi's brother Cao Ju (曹據), the Prince of Pengcheng, emperor, she managed to persuade him that such a succession would be improper, since Cao Ju was the uncle of her husband Cao Rui, such a succession would leave Cao Rui effectively without an heir. Sima Shi was forced to agree with her to let Cao Mao be the emperor. (Cao Mao, although young (at age 13) was known for his intelligence, and Empress Dowager Guo might have believed that he, alone of the princes and dukes, might have had a chance of counteracting the Simas.) When Sima Shi asked her for the imperial seal, she again reasoned with him and refused politely, under the reasoning that she had met Cao Mao before and wanted to personally hand him the seal. When Cao Mao was summoned to the capital, he acted in accordance with the ceremonies due a duke, rather than putting on imperial pretensions immediately, until he was enthroned. This earned him popular support and praise as a humble young emperor.

In 255, generals Guanqiu Jian and Wen Qin started a rebellion against the Sima clan in Shouchun, but were quickly crushed by Sima Shi's forces. Guanqiu was killed, and his clan was slaughtered. Wen and his sons, Wen Yang and Wen Hu, fled to the rival state Eastern Wu. Sima Shi died of illness shortly after the rebellion was suppressed. In the aftermath of Sima's death, the 14-year-old Cao Mao made another effort to seize back state power. He issued an imperial edict for Sima Shi's successor Sima Zhao to remain at Xuchang, using an excuse that the situation at Shouchun was still not completely peaceful. He also recalled Sima's assistant Fu Gu to return to the capital with the troops. However, on the advice of Fu and Zhong Hui, Sima Zhao ignored the edict and returned to Luoyang, and continued to remain in control of the government.

Over the next few years, Cao Mao gradually established a circle of people around him, including Sima Zhao's cousin Sima Wang, Wang Shen (王沈), Pei Xiu, and Zhong Hui, all of whom were known for their literary talent. These people were unquestioned in their support for the Sima clan, but they might also have something to gain if they pledged allegiance to Cao Mao. In doing so, Cao was hoping that he could reduce Sima Zhao's suspicions against him while winning support from these people. He often held meetings with them to discuss literature. In addition, he gave Sima Wang a fast two-wheeled wagon and five imperial guardsmen as escorts because Sima lived further away from the palace than the others.

Around 257, Zhuge Dan, who replaced Guanqiu Jian as military commander in Shouchun, started a rebellion against Sima Zhao, with support from the rival state of Eastern Wu. Sima led an army to suppress the revolt and trapped the rebels in the city by early 258. Internal conflict broke out in Shouchun between Zhuge and Wen Qin (returned from Eastern Wu to support the revolt), which concluded with Wen's death at Zhuge's hands and the defection of Wen's sons to Sima Zhao. Shouchun eventually fell to Sima's forces and the rebellion was effectively crushed. In 259, Cao Mao received reports of sightings of yellow dragons (a sign of divine favour) in two wells. He commented that it was actually a sign of divine disfavour, and wrote a poem titled Ode to the Hidden Dragon (濳龍詩):

The poor dragon is trapped, alone and cold;

He cannot leap out of the depths;

He cannot rise to the heavens;

He cannot even descend onto fields.

The poor dragon fell into the deep well;

Even catfish dance before him;

He hides his teeth and claws and sighs;

And I am this depressed as well?

The poem greatly displeased Sima Zhao, who paid more attention to Cao Mao's activities afterwards. In 258, under pressure from Sima, Cao issued an edict granting Sima the nine bestowments, but Sima declined.

In 260, Cao Mao was again forced to issue an edict granting Sima Zhao title as Duke of Jin and the nine bestowments, which Sima Zhao did not decline. Cao Mao gathered his associates Wang Shen, Wang Jing, and Wang Ye, and announced his plan for a last effort against Sima Zhao to regain his power. Wang Shen and Wang Ye went to warn Sima Zhao. Personally armed with a sword, Cao Mao led his personal guard to go from the Palace to attack Sima Zhao. Sima Zhao's brother Sima Zhou led a defense at a gate but was routed. Jia Chong led another defense at the southern watchtower, and ordered his officer Cheng Ji to kill Cao Mao. Cheng Ji stabbed Cao Mao in the chest with a spear, killing him.

Sima Zhao forced the Empress Dowager Guo to publish an order accusing Cao Mao of plotting to attack the Empress Dowager and posthumously strip Cao Mao of rank, in order to present the attack as aimed primarily against the Empress Dowager instead of himself. Sima Zhao with his uncle Sima Fu and the other high ministers then memorialized to re-instate Cao Mao as a Duke and nominally bury him as a prince for appearances of leniency. The burial was not actually done following the rites of a prince. Sima Zhao also retroactively declined his title as Duke of Jin and the nine bestowments.

Jia Chong's death was called for on account of regicide, but it was Cheng Ji himself who was executed for the crime 19 days after the emperor's death. Cao Huang (later renamed to Cao Huan), the Duke of Changdao, was instated as emperor as a result, the last puppet emperor before Sima Yan would usurp the imperial throne.